tools/kvm_stat: eliminate extra guest/pid selection dialog

We can do with a single dialog that takes both, pids and guest names.
Note that we keep both interactive commands, 'p' and 'g' for now, to
avoid confusion among users used to a specific key.

While at it, we improve on some minor glitches regarding curses usage,
e.g. cursor still visible when not supposed to be.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>