kvm: Allow the Hyper-V vendor ID to be specified

According to Microsoft documentation, the signature in the standard
hypervisor CPUID leaf at 0x40000000 identifies the Vendor ID and is
for reporting and diagnostic purposes only.  We can therefore allow
the user to change it to whatever they want, within the 12 character
limit.  Add a new hv-vendor-id option to the -cpu flag to allow
for this, ex:

 -cpu host,hv_time,hv-vendor-id=KeenlyKVM

Link: http://msdn.microsoft.com/library/windows/hardware/hh975392
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Message-Id: <20151016153356.28104.48612.stgit@gimli.home>
[Adjust error message to match the property name, use error_report. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
