Add KVM machine version as hv param

This is needed in case an instance must be booted with an exact type of
machine version (due to e.g. outdated drivers). If this hv param
explicitly defined (during instance creation/modification) then the kvm
process will be started having this version for the -M option. Otherwise
the default (derived from _GetDefaultMachineVersion() will be used.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Guido Trotter <ultrotter@google.com>