VirtualDomain: avoid running "virsh uri" if hypervisor is set

Recent efforts to make this RA independent of a temporarily stopped
or blocking libvirtd missed the unconditional call to "virsh uri"
use to determine the hypervisor default. Since
OCF_RESKEY_hypervisor_default is actually not used anymore, we can
invoke virsh uri directly when setting OCF_RESKEY_hypervisor, but
only if the latter isn't already set.

Thus, users can avoid the superfluous "virsh uri" call altogether
by explicitly setting the hypervisor RA parameter.
VirtualDomain: check process table for qemu-system-* too, not just qemu-kvm

The Qemu/KVM process name "qemu-kvm" is deprecated; qemu-system-<arch>
is now the preferred binary name. Some distributions only support the
latter. Expand the regular expression in monitor to support both names
when grepping through the process table.

Use an extended regular expression to test for both qemu-kvm
and qemu-system-*; the -E flag is specified by POSIX.