x86/hvm: Crash domain rather than guest on unexpected PIO IO state

x86/hvm: Crash domain rather than guest on unexpected PIO IO state

Under certain conditions, if an IO gets into an unexpected state,
hvmemul_do_io can return X86EMUL_UNHANDLEABLE.  Unfortunately,
handle_pio() does not expect this state, and calls BUG() if it sees
it, crashing the host.

Other HVM io-related code crashes the guest in this case.  This patch
makes handle_pio() do the same.

The crash was seen when executing crash_guest in dom0 to forcibly
crash the guest.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>