Move the boot validation being done by qemuBuildBootCommandLine()
to to a new qemuDomainDefValidateBoot() function. This new function
is called by qemuDomainDefValidate(), allowing boot validation in
domain define time.

Tests were adapted to consider the new caps being needed in
this earlier stage.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>