gdbstub: reject unsupported flags in handle_set_qemu_sstep

handle_query_qemu_sstepbits is reporting NOIRQ and NOTIMER bits
even if they are not supported (as is the case with record/replay).
Instead, store the supported singlestep flags and reject
any unsupported bits in handle_set_qemu_sstep.  This removes
the need for the get_sstep_flags() wrapper.

While at it, move the variables in GDBState, instead of using
global variables.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
[Extracted from Maxim's patch into a separate commit. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211111110604.207376-4-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>