serial: check if backed by a physical serial port at realize time

Right now, s->poll_msl may linger at "0" value for an arbitrarily long
time, until serial_update_msl is called for the first time.  This is
unnecessary, and will lead to the s->poll_msl field being unnecessarily
migrated.

We can call serial_update_msl immediately at realize time (via
serial_reset) and be done with it.  The memory-mapped UART was already
doing that, but not the ISA and PCI variants.

Regarding the delta bits, be consistent with what serial_reset does when
the serial port is not backed by a physical serial port, and always clear
them at reset time.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>