Description of problem:
When creating a new Host and specifying on Virtual Machine tab "Allocation(GB)" as empty, Host create fails with

NoMethodError: undefined method `[]' for nil:NilClass

error. It is more user-unfriendly also due to the fact that the Allocation(GB) is purged when one clicks to "Size" button right from the value.


Version-Release number of selected component (if applicable):
Sat 6.3.3
Sat 6.4 beta (checked it's also there)


How reproducible:
100%


Steps to Reproduce:
1. Start creating a new Host
2. In Virtual Machine tab, click to "Size" button (or leave "Allocation(GB)" empty either way)
3. Fill all other parameters properly.
4. Submit the Host


Actual results:
Host create fails with "undefined method `[]' for nil:NilClass"


Expected results:
Either no error (if the parameter can have empty value with some meaning), or provide some meaningfull error message before tryng to create the host (e.g. similar to leaving root password empty).


Additional info:
backtrace:
2018-09-04 15:20:37 18507e4a [app] [W] Failed to create a compute satotest (Libvirt) instance pmoravec-test.sysmgmt.lan: undefined method `[]' for nil:NilClass

 NoMethodError: undefined method `[]' for nil:NilClass
 /opt/theforeman/tfm/root/usr/share/gems/gems/fog-libvirt-0.2.0/lib/fog/libvirt/models/compute/volume.rb:119:in `split_size_unit'
 (erb):3:in `to_xml'

Relevant code:

112         def split_size_unit(text)
113           if text.kind_of? Integer
114             # if text is an integer, match will fail
115             size    = text
116             unit    = 'G'
117           else
118             matcher = text.match(/(\d+)(.+)/)
119             size    = matcher[1]
120             unit    = matcher[2]
121           end
122           [size, unit]
123         end

that simply does not count with empty value in text