libvirt: optimize pause mode support

Commit bfb28fcf9031af4c695177663702ce05edbbfa4d added a method
call that checks if the libvirt driver support the
VIR_DOMAIN_START_PAUSED flag. This patch optimizes that by reading
the value at startup.

Change-Id: I0fc1edec55df609ab25b9999f4a0d077ddcf0be7
Merge "libvirt: optimize pause mode support"