tests: Mock access to /dev/kvm

tests: Mock access to /dev/kvm

Some of our tests try to validate domain XMLs they are working
with (not intentionally, simply because they call top level
domain XML parse function). Anyway, this implies that we build
domain capabilities also - see
virQEMUDriverGetDomainCapabilities(). And since some domain XMLs
are type of 'kvm' the control gets through
virQEMUCapsFillDomainCaps() and virHostCPUGetKVMMaxVCPUs() to
opening /dev/kvm which may be missing on the machine we're
running 'make check'.

Previously, we did not see this issue, because it was masked. If
building domain capabilities failed for whatever reason, we
ignored the failure. Only v5.9.0-207-gc69e6edea3 uncovered the
problem (it changed reval from 0 to -1 if
virQEMUDriverGetDomainCapabilities() fails). Since the referenced
commit is correct, we need to mock access to /dev/kvm in our
tests.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>