core/hvm: fix error message when creating HVM while not supported by the hardware
core/hvm: fix error message when creating HVM while not supported by the hardware

libvirt reports such errors at domain definition, not startup (as was in
libxl).

QubesOS/qubes-issues#1198

woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin