libvirt: pause mode is not supported by all drivers

Only KVM/Qemu drivers support the VIR_DOMAIN_START_PAUSED flag
Booting guests on other drivers with the above flag will make it fail.

Closes-Bug: 1301453
Change-Id: Ia98e018b686c4ec3c15fd1f6bcc78188f330fcef
(cherry picked from commit bfb28fcf9031af4c695177663702ce05edbbfa4d)