Fix up basic migration.

Fix up basic migration.

Basic live migration was broken by the commit that added
non-shared block support in two ways:

1)  It added a virCheckFlags() to doNativeMigrate().  Besides
the fact that typical usage of virCheckFlags() is in driver
entry points, and doNativeMigrate() is not an entry point,
it was missing important flags like VIR_MIGRATE_LIVE.  Move
the virCheckFlags to the top-level qemuDomainMigratePrepare2
and friends.

2)  It also added a memory leak in qemuMonitorTextMigrate()
by not freeing the memory used by virBufferContentAndReset().
This is fixed by storing the pointer in a temporary variable
and freeing it at the end.

With this patch in place, normal live migration works again.

v3: Instead of the churn for virCheckFlagsUI and UL, instead
always promote flags to an unsigned long and always use %lx
for the fprintf.
v2: Add back flags check, which required adding virCheckFlagsUI
and virCheckFlagsUL

Signed-off-by: Chris Lalancette <clalance@redhat.com>