fakevirt: return hypervisor_version as an int instead of a string

The compute_nodes.hypervisor_version type in the database is an integer
not a string. This is only caught in postgres and not MySQL. Changing
the fakevirt driver to return the proper type allows for the
test_virt_drivers test to make check this for all drivers it currently
tests.

Change-Id: I25e4dba855b954a2451bced9a53172b51eb47238
Merge "fakevirt: return hypervisor_version as an int instead of a string"