Fix ZTS MMP tests and ztest -M behavior

Fix ZTS MMP tests and ztest -M behavior

Quote "$MMP_IMPORT_MSG" when it is passed as an argument, as it is a
multi-word string.  Some tests were passing when they should not have,
because the grep was only testing for the first word.

Correct the message expected when no hostid is set and the test attempts
to enable multihost.  It did not match the actual output in that
situation.

Disable ztest_reguid() when ztest is invoked with the -M option.  If
ztest performs a reguid, a concurrent import attempt may fail with the
error "one or more devices is currently unavailable" if the guid sum is
calculated on the original device guids but compared against the guid
sum ztest wrote based on the new device guids.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #6666