whpx: Added support for breakpoints and stepping

Below is the updated version of the patch adding debugging support to WHPX.
It incorporates feedback from Alex Bennée and Peter Maydell regarding not
changing the emulation logic depending on the gdb connection status.

Instead of checking for an active gdb connection to determine whether QEMU
should intercept the INT1 exceptions, it now checks whether any breakpoints
have been set, or whether gdb has explicitly requested one or more CPUs to
do single-stepping. Having none of these condition present now has the same
effect as not using gdb at all.

Message-Id: <0e7f01d82e9e$00e9c360$02bd4a20$@sysprogs.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>