esx: Support for disk-only and quiescing snapshots.

esx: Support for disk-only and quiescing snapshots.

Add support for creating disk-only (no memory) snapshots in esx, and
for quiescing the VM before taking the snapshot. The VMware API
supports these operations directly, so adding support to libvirt is
just a matter of setting the flags correctly when calling
VMware. VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY and
VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE are now valid flags for esx.

Signed-off-by: Eric Blake <eblake@redhat.com>