qemu: hotplug: Simplify removal of managed PR infrastructure on unplug

qemu: hotplug: Simplify removal of managed PR infrastructure on unplug

Extract the (possible) removal of the PR backend and daemon into a
separate helper which enters monitor on its own. This simplifies the
code and allows reuse of this function in the future e.g. for blockjobs
where removing a image with PR may result into PR not being necessary.

Since the PR is not used often the overhead of entering monitor again
should be negligible.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>