tests/migration-test: Silence the kvm_hv message by default

tests/migration-test: Silence the kvm_hv message by default

When running "make check" on a non-POWER host, there is currently an ugly
line in the output like this:

  [...]
  GTESTER check-qtest-nios2
  GTESTER check-qtest-or1k
  GTESTER check-qtest-ppc64
Skipping test: kvm_hv not available Skipping test: kvm_hv not available Skipping test: kvm_hv not available Skipping test: kvm_hv not available   GTESTER check-qtest-ppcemb
  GTESTER check-qtest-ppc
  GTESTER check-qtest-riscv32
  GTESTER check-qtest-riscv64
  [...]

Move the check to the beginning of the main function instead, so that
we do not have to test the condition again and again for each test,
and better use g_test_message() instead of g_print() here, like it is
also done in ufd_version_check() already.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>