Description of problem:
virt-manager can't boot from ISO on readonly NFS
Unable to complete install: 'unable to set security context 'system_u:object_r:virt_content_t:s0'

Version-Release number of selected component (if applicable):
virt-manager-0.10.0-7.el7.noarch
selinux-policy-3.12.1-103.el7.noarch
selinux-policy-targeted-3.12.1-103.el7.noarch

How reproducible:
always

Steps to Reproduce:
1. Create a new VM, select local media install.
2. Add ISO on read-only NFS as installation source.
3. Finish VM setup.

Actual results:
Unable to complete install: 'unable to set security context 'system_u:object_r:virt_content_t:s0' on '/mnt/globalsync/fedora/linux/releases/19/Fedora/x86_64/iso/Fedora-19-x86_64-DVD.iso': Read-only file system'

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 100, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/create.py", line 1920, in do_install
    guest.start_install(False, meter=meter)
  File "/usr/share/virt-manager/virtinst/Guest.py", line 1143, in start_install
    noboot)
  File "/usr/share/virt-manager/virtinst/Guest.py", line 1211, in _create_guest
    dom = self.conn.createLinux(start_xml or final_xml, 0)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 2892, in createLinux
    if ret is None:raise libvirtError('virDomainCreateLinux() failed', conn=self)
libvirtError: unable to set security context 'system_u:object_r:virt_content_t:s0' on '/mnt/globalsync/fedora/linux/releases/19/Fedora/x86_64/iso/Fedora-19-x86_64-DVD.iso': Read-only file system