x86/HVM: handle_{mmio*,pio}() return value adjustments

x86/HVM: handle_{mmio*,pio}() return value adjustments

Don't ignore their return values. Don't indicate success to callers of
handle_pio() when in fact the domain has been crashed.

Make all three functions return bool. Adjust formatting of switch()
statements being touched anyway.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>