Bug 1146339

Description of problem:
Improve the error showing when no enough space left on host while conversion

Version-Release number of selected component (if applicable):
virt-v2v-1.27.54-1.1.el7.x86_64
libguestfs-1.27.54-1.1.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Copy xen guest image and xml file from xen server to v2v server:
# qemu-img info /var/lib/xen/images/rhel6.6-pv-x64-test.img
image: /var/lib/xen/images/rhel6.6-pv-x64-test.img
file format: raw
virtual size: 7.8G (8388608000 bytes)
disk size: 7.8G

2. Check the host left space <8G
# df -h 
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda6        98G   95G  3.4G  97% /
devtmpfs        3.8G     0  3.8G   0% /dev
tmpfs           3.9G  144K  3.9G   1% /dev/shm
tmpfs           3.9G  169M  3.7G   5% /run
tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup
/dev/sda1        96G   88G  3.6G  97% /mnt
/dev/sda3        98G   39G   59G  40% /opt

3. Run virt-v2v to connect a xen pv/hvm guest either by -i disk or -i libvirtxml.
# export LIBGUESTFS_BACKEND=direct

# virt-v2v -i libvirtxml -o local -os  /var/tmp/  rhel6.6-pv-x64-test.xml -on test33 -of raw -oa preallocated
[   0.0] Opening the source -i libvirtxml rhel6.6-pv-x64-test.xml
[   0.0] Creating an overlay to protect the source from being modified
[   1.0] Opening the overlay
[   4.0] Initializing the target -o local -os /var/tmp/
[   4.0] Inspecting the overlay
[   8.0] Checking for sufficient free disk space in the guest
[   8.0] Estimating space required on target for each disk
[   8.0] Converting Red Hat Enterprise Linux Server release 6.6 Beta (Santiago) to run on KVM
This guest has virtio drivers installed.
[  34.0] Mapping filesystem data to avoid copying unused and blank areas
[  35.0] Closing the overlay
[  35.0] Copying disk 1/1 to /var/tmp/test33-sda (raw)
virt-v2v: error: libguestfs error: /var/tmp/test33-sda: fallocate: No space 
left on device

If reporting bugs, run virt-v2v with debugging enabled and include the 
complete output:

  virt-v2v -v -x [...]

4. Run step3 again with host space using almost 100%.
# df -h 
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda6        98G   98G   20K 100% /
devtmpfs        3.8G     0  3.8G   0% /dev
tmpfs           3.9G  144K  3.9G   1% /dev/shm
tmpfs           3.9G  161M  3.7G   5% /run
tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup
/dev/sda1        96G   88G  3.6G  97% /mnt
/dev/sda3        98G   39G   59G  40% /opt

# virt-v2v -i libvirtxml -o local -os  /var/tmp/  rhel6.6-pv-x64-test.xml -on test333 -of raw -oa preallocated 
[   0.0] Opening the source -i libvirtxml rhel6.6-pv-x64-test.xml
[   0.0] Creating an overlay to protect the source from being modified
virt-v2v: error: /tmp/v2vovl8aad70.qcow2: No space left on device

If reporting bugs, run virt-v2v with debugging enabled and include the 
complete output:

  virt-v2v -v -x [...]

Actual results:
As described.

Expected results:
"fallocate" is a function name, didn't show such function in error message, it's unsafe and unfriendly.
step3 and step4 result should keep consistent.

Additional info:
Attached log file for step3 and step4.