migration: equation is more proper than and to check LOADVM_QUIT

migration: equation is more proper than and to check LOADVM_QUIT

LOADVM_QUIT allows a command to quit all layers of nested loadvm loops,
while current return value check is not that proper even it works now.

Current return value check "ret & LOADVM_QUIT" would return true if
bit[0] is 1. This would be true when ret is -1 which is used to indicate
an error of handling a command.

Since there is only one place return LOADVM_QUIT and no other
combination of return value, use "ret == LOADVM_QUIT" would be more
proper.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190718064257.29218-1-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>